Liquid crystal display devices and organic EL display devices, which include a thin film transistor (TFT) for each pixel, have been in wide use in recent years. TFTs are manufactured using a semiconductor layer formed on a substrate, such as a glass substrate. A substrate on which the TFTs have been formed is called an active matrix substrate.
As such TFTs, TFTs with an amorphous silicon film as the active layer (amorphous silicon TFTs) and TFTs with a polysilicon film as the active layer (polysilicon TFTs) have been widely used.
Because the carrier mobility in a polysilicon film is higher than in an amorphous silicon film, polysilicon TFTs offer higher ON currents and operate faster than amorphous silicon TFTs. For this reason, display panels having polysilicon TFTs not only for the pixels, but also for part or all of peripheral circuits, such as a driver, are under development.
Manufacturing of the polysilicon TFTs, however, requires complicated steps, including a laser crystallization step for crystallizing an amorphous silicon film, a thermal anneal step, and an ion doping step, which contribute to higher manufacturing cost per substrate unit area. For this reason, polysilicon TFTs are mainly used today for mid- to small-sized display devices, and amorphous silicon TFTs are used for large display devices.
As larger display devices have become available in recent years, amidst increasing demand for higher image quality and lower power consumption, TFTs with a microcrystalline silicon (μc-Si) film as the active layer, which offer higher performance and lower manufacturing cost than the amorphous silicon TFTs, have been proposed (Patent Document 1, Patent Document 2, and Non-Patent Document 1). Such TFTs are called microcrystalline silicon TFTs.
A microcrystalline silicon film is a silicon film having crystal and amorphous phases, and is constructed of microcrystalline grains spread throughout an amorphous matrix. The size of each microcrystalline grain (several hundred nanometers or less) is smaller than the size of crystal grains in a polysilicon film. The microcrystalline grains may be columnar crystals.
A microcrystalline silicon film may be formed, for example, using a plasma CVD method, and does not require an anneal process for crystallization. Thus, it can be formed using a conventional manufacturing facility for amorphous silicon films. Furthermore, because a microcrystalline silicon film offers a higher carrier mobility than an amorphous silicon film, a microcrystalline silicon TFT offers higher performance than an amorphous silicon TFT.
For example, Patent Document 1 describes that use of a microcrystalline silicon film as the TFT active layer would yield ON currents that are 50% higher than that of the amorphous silicon TFTs. Furthermore, Non-Patent Document 1 describes that use of a semiconductor film made of microcrystalline silicon and amorphous silicon yields TFTs having an on/off current ratio of 106, a mobility of about 1 cm2/Vs, and a threshold voltage of about 5 volt.
Furthermore, Patent Document 2 discloses an inverse staggered TFT made of microcrystalline silicon.
In spite of offering many advantages described above, microcrystalline silicon TFTs have yet to be commercialized to date. One of the reasons is that microcrystalline silicon TFTs have high OFF currents (leakage currents).
A possible approach to reducing the TFT OFF current is to introduce a multi-gate structure already in use for polysilicon TFTs. For example, Patent Documents 3 and 4 disclose liquid crystal display devices and organic EL display devices using microcrystalline silicon TFTs having multi-gate structures.